Common Reef Creatures – Invertebrates

Here are some common creatures you will see whilst snorkelling on the reefs around the Mamanucas. There is also a description of the organism and its role in the ecosystem. Every creature has its place in the marine environment so please respect them and leave them where they are.

Part 1—Invertebrates

Invertebrates are a group of animals that have no backbone.

Sea Stars

The Blue Sea star (Linkia laevigata) is a common inhabitant of the coral reef. There are approximately 1,500 species of sea stars in the world. They use a mouth on the underside of their body to feed on ocean bottom debris, as well as mussels and clams. Sea stars are capable of regenerating body parts from body fragments. For example, if a starfish were cut in half, two whole starfish would grow from both halves. Because of the variety of food the starfish consumes, it plays a very important role in maintaining the diversity of the food chain.

Image: Blue Sea Star

Sea Cucumbers

In marine environments such as those in Fiji, many brightly coloured plants and animals often overshadow other, less ostentatious creatures, such as the sea cucumber. They move slowly (3 inches/hour) along the ocean floor sucking up sand and debris. Incredibly, over the span of a year, this creature can vacuum up to 200 pounds of sand! These invertebrates play an important role in filtering sediments and recycling nutrients back into the food web. Likewise, the eggs of sea cucumbers provide nutrients for many marine species. Therefore the significance of these curious looking creatures cannot be underestimated. In Asia the sea cucumber is considered a gourmet delicacy and is collected from other parts of the world such as Fiji for this market. Because they grow and mate slowly, populations of sea cucumbers can be rapidly depleted due to over harvesting, upsetting the balance in the ecosystem.

Image: Black Sea Cucumber (Beche-de-Mer)

Giant Clams

There are 9 species of giant clams and they mostly all have very colourful mantles. Previously Tridacna gigas (The King of the giant clams) was prevalent in Fiji waters but has been over fished and is virtually extinct. These clams can grow up to 140cm and the largest one known weighed in at 734 pounds or 333kg! Giant clams are excellent bio-filters, which mean they extract nutrients out of the water. Their brilliant colours are due to a symbiotic relationship with algae in their tissue. It was once thought that the Giant Clam could trap a diver underwater by closing suddenly on his or her foot, but this could only happen to a very slow or very careless diver!

Image: Fluted Giant Clam (Tridacna Squamosa)

Sea Urchins

These non aggressive individuals don’t move very fast. But if you accidentally step on one of them, you undoubtedly will! Sea urchins are pincushion like creatures (allied to the starfish) that populate reefs worldwide. Many species have short, blunt spines. Others sport long, sharp spines, with very sharp tips, which act as natural hypodermic needles. They are generally brittle and break off easily beneath the skin. Toxin injected by the urchin’s venom glands can cause intense pain. Sea urchins have a significant role on the reef as they graze on algae. This leaves room and gives the chance for animals such as corals to establish themselves and grow.

Image: Long-spined sea urchin (Diadema spp)

Cone Shells

There are about 500 species of the prettily marked cone shells, all of which are predatory animals. Most cone shells live on worm or on other molluscs, including snails of their own kind. There are about 70 species that feed on fish. These often beautiful creatures possess a well-developed venom apparatus, a minute harpoon, called a radula tooth, which can inject powerful venom. Around twenty cones are known to be dangerous to humans and a few of them are lethal. The bottom line is, avoid handling cone shells! They may look attractive, but many are capable of inflicting a fatal sting capable of penetrating gloves, clothing, or even a thin wetsuit.